Mopar Muscle did the Stealth Bomber Diplomat build and out a warmed 360 in it with headers and went, I think, 3.5" to the muffler and 3" after to the back.

If I were you I would go 3" single all the way back. If a dual 2.5" will work and its in a kit form then use that, it won't hurt your gas mileage but will improve power. With a cam 4bbl and headers it could use at least a 3" single